The present invention relates to a process for preparing bis(para-aminocyclohexyl)methane by multiphase reaction of methylenedianiline with hydrogen, in which the reaction is performed in 5 to 50 series-connected reaction zones under adiabatic conditions.
Bis(para-aminocyclohexyl)methane (PACM) is generally prepared under the catalytic influence of transition metal catalysts, for instance ruthenium catalysts or rhodium catalysts, from a liquid phase comprising methylenedianiline and a gaseous phase comprising hydrogen in an exothermic catalytic reaction according to Formula (I):

The PACM prepared by means of the reaction according to Formula (I) is an intermediate in the preparation of polyurethane, which in turn possesses a wide range of possible uses.
For instance, the resulting polyurethane finds use as a base for coatings. Alternatively, it can be foamed and thus used as polyurethane foam, for instance as an insulating material or as a cushioning material.
The reaction according to Formula (I) releases a considerable amount of heat which at first is absorbed in the liquid phase and has to be removed therefrom again in order to prevent, for instance, evaporation of the liquid phase. Moreover, the temperature of such reactions must be controlled in order to prevent the possibility of explosive side reactions, as known to the person skilled in the art in connection with aromatic chemistry.
U.S. Pat. No. 6,998,507 B1 discloses a process for preparing PACM, which also comprises the preparation of the methylenedianiline in a preliminary stage. The preparation of PACM according to the subject matter of this invention is performed in a reactor which comprises a monolithic catalyst in a stirred vessel. The catalyst is preferably a catalyst composed of a rhodium/ruthenium bimetallic catalyst.
The disclosure of U.S. Pat. No. 6,998,507 B1 does not include any indication to a possible adiabatic mode of operation of the reaction zone, although precooling of the methylenedianiline before entry into the reaction zone for preparing PACM is disclosed.
The process is disadvantageous because, in the case of failure of this only simple cooling provided according to U.S. Pat. No. 6,998,507 B1, the reaction runs away in an uncontrolled manner and the operating states with risk of explosion outlined above can be attained. Reliable and exact temperature control is, moreover, also not possible with the process disclosed in U.S. Pat. No. 6,998,507 B1 merely because considerable volumes of the reaction zone are not in direct contact with a heat-exchanging surface, such that the formation of uncontrolled temperature gradients within the reaction zone is probable, which probably also constitutes the reason for the essentially batchwise mode of operation according to the process of U.S. Pat. No. 6,998,507 B1.
A further process for general hydrogenations, including the hydrogenation of methylenedianiline to PACM according to the Formula (I) being considered here, is disclosed by EP 1 566 372 A1, which discloses trickle bed arrangements in tubes in the process disclosed here. The reaction zones which arise as a result may be filled with different catalyst materials. EP 1 566 372 further discloses that only upstream and/or downstream of a number of three series-connected reaction zones can cooling be effected.
There is no specific disclosure regarding an adiabatic or isothermal mode of operation in EP 1 566 372 A1. For the lack of a disclosure regarding thermal insulation around the reaction zones and in view of the high surface-to-volume ratio of the tube arrangement according to EP 1 566 372 A1, it can be assumed that it is desired to remove heat via the surfaces of the tubes, which leads, if anything, to a polytropic mode of operation of the process.
The process according to the disclosure of EP 1 566 372 A1 is disadvantageous, since, just as in the case of the process according to U.S. Pat. No. 6,998,507 B1, unsafe operating states of the strongly exothermic reaction according to the Formula (I) cannot be controlled sufficiently, since cooling of the reactants is provided only upstream and/or downstream of a multitude of reaction zones. Moreover, the heat of reaction, for the lack of measures for absorption thereof, is dissipated into the environment and is thus lost, which is uneconomic. A further factor associated with the lack of temperature control of the process according to EP 1 566 372 A1 is that the desired selectivities and yields of PACM can be complied with only with difficulty when there is operating disruption.
A third process for preparing PACM is disclosed by U.S. Pat. No. 5,196,594 A, according to which PACM among other substances can be formed from methylenedianiline with hydrogen in at least one fixed bed reactor at temperatures of 100° C. to 190° C. under a pressure of 50 to 350 bar.
The maximum number of reaction zones usable in series according to the disclosure of U.S. Pat. No. 5,196,594 A is two, and no cooling between the reaction zones is disclosed. Moreover, no specific disclosure regarding the mode of operation is given, from which a distinction between a preferably isothermal or adiabatic procedure could have been drawn.
The process according to U.S. Pat. No. 5,196,594 A has, however, at least the disadvantages of the processes according to the disclosures of EP 1 566 372 A1 and U.S. Pat. No. 6,998,507 B1, since sufficient temperature control cannot be achieved here either, which in turn leads at least to an endangerment potential or to insufficient yields/selectivities.
EP 1 251 951 (B1) discloses an apparatus and the possibility of performing chemical reactions in the apparatus, the apparatus being characterized by a cascade of reaction zones and heat exchanger apparatuses which are in contact with one another and are arranged cohesively bonded to one another in an integrated system. The process to be performed here is thus characterized by the contact of the different reaction zones with a particular heat exchanger apparatus in the form of a cascade.
There is no disclosure with regard to the usability of the apparatus and of the process for synthesis of PACM from liquid methylenedianiline and gaseous hydrogen. More particularly, applicability to polyphasic reaction systems in general is not disclosed.
It thus remains unclear how, proceeding from the disclosure of EP 1 251 951 (B1), such a reaction is to be performed by means of the apparatus and of the process performed therein. Moreover, for reasons of unity of invention, it has to be assumed that the process disclosed in EP 1 251 951 (B1) is performed in an apparatus identical or similar to the disclosure with regard to the apparatus. The result of this is that, due to the large-area contact of the heat exchange zones with the reaction zones according to the disclosure, a significant amount of heat is transferred by conduction of heat between the reaction zones and the adjacent heat exchange zones. The disclosure with regard to the oscillating temperature profile can thus only be understood such that the temperature peaks found here would be greater if this contact were not to exist. A further indication of this is the exponential rise in the temperature profiles disclosed between the individual temperature peaks. These indicate that a certain heat sink with notable but limited capacity is present in each reaction zone, which can reduce the temperature rise therein. It can never be ruled out that a certain removal of heat (for example by radiation) takes place; however, in the event of a reduction in the possible heat removal from the reaction zone, there would be an indication of a linear temperature profile or of one with declining slope, since no further metering of reactants is intended, and thus, after exothermic complete reaction, the reaction would become ever slower and the exothermicity generated would thus be reduced. Thus, EP 1 251 951 (B1) discloses multistage processes in cascades of reaction zones, from which heat is removed in an undefined amount by conduction of heat. Accordingly, the process disclosed is disadvantageous in that exact temperature control of the process gases of the reaction is impossible.
Proceeding from the prior art, it would therefore be advantageous to provide a process which can be performed in simple reaction apparatus and which enables exact, simple temperature control, such that it allows high conversions with maximum purities of the PACM product.
For the preparation of PACM from methylenedianiline by means of catalytic hydrogenation, as just described, no suitable processes capable of achieving the aforementioned objects in their entirety have been disclosed to date.
It is therefore an object of the invention to provide a process for catalytically hydrogenating methylenedianiline to PACM, which is performable with exact temperature control in simple reaction apparatus and which, as a result, allows high conversions at high purities of the product, the heat of reaction being utilizable either for the benefit of the reaction or in another way.